The present invention relates to turbochargers having a centrifugal compressor driven by the exhaust gases from an internal combustion engine and, more particularly, to an assist system to inject a power stream of auxiliary driving fluid which acts on the compressor wheel until the driving power of the exhaust gases is sufficient.
Turbochargers for internal combustion engines include a centrifugal compressor mounted on a common shaft with a turbine. The high pressure exhaust from the internal combustion engine drives the turbine such that the rotating turbine causes the compressor wheel of the compressor to rotate. The air entering the inlet of the compressor is then compressed by the compressor wheel which discharges the pressurized air into the inlet of the internal combustion engine thereby resulting in an improved efficiency for the engine. In this manner, the turbocharger transfers energy from the engine exhaust gas to the engine's intake air.
During the start-up of the internal combustion engine, the energy available from the exhaust gases is insufficient to effectively drive the turbocharger. Therefore, to obtain rapid acceleration of the engine or to obtain a better operation of the engine under accelerating conditions the turbocharger includes an air start/assist system which directs a stream of auxiliary driving fluid such as compressed air onto the vanes or blades of the compressor wheel to generate increased torque on the compressor wheel to thereby increase the speed of rotation of the wheel. An air manifold or distribution chamber extends around the compressor housing as a compressed air supply and the housing includes a plurality of nozzles shaped so that they direct a jet of compressed air at an angle to the vanes or blades.
U.S. Pat. No. 3,462,071 discloses a turbocharger compressor which includes nozzles for directing an auxiliary propellant against the trailing faces of the blades. Tangential feed lines provide auxiliary propellant to the compressor manifold and the housing includes a plurality of bladed nozzles directed toward the blades of the compressor wheel.
U.S. Pat. No. 4,689,960 discloses a compressor housing having an odd number of nozzles directed toward the impeller at an angle. The angle causes the axis of the nozzles to be in a plane parallel to the axis of the compressor wheel and at an angle of 15 to 25 degrees in that plane. Alternatively, the axis may be in a plane which is inclined with respect to the axis of rotation of the compressor wheel so that the compressed air will flow in a preferred direction towards the outer edge of the wheel.
U.S. Pat. No. 4,696,165 discloses a plurality of nozzles positioned around the entire circumference of the impeller casing.
Prior art systems have the disadvantage that undue stresses are placed on the blades resulting from the high pressure air jets acting on the impeller blades. These stresses cause the blades to oscillate at their natural frequency, thereby shortening blade life and resulting in dangerous vibrations. Hence, an assist system is desired which applies maximum force to the compressor wheel while minimizing the adverse oscillation and uneven loading on the blades.
Further, prior art assist systems teach a minimum number of nozzles or an uneven distribution of nozzles around the compressor wheel so as to provide an uneven loading of forced air onto individual blades as the blades rotate on the compressor wheel. Also, the nozzles are directed radially such that the compressed air can either impinge on the base of the compressor wheel between adjacent blades or exit between the blades without impinging on the wheel at all, or the nozzles are directed tangentially causing the air to flow towards the hub of the compressor wheel thereby losing efficiency. Further, the number of nozzles, the geometry and angles of the nozzles of the prior art do not provide optimum efficiency for the air start/assist system of the turbocharger.